Oxygen isotope composition of phloem sap in relation to leaf water in Ricinus communis

We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The ¹⁸O / ¹⁶O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of α_o=1.027, where α_o=R_o / R_w , and R_o and R_w are ¹⁸O / ¹⁶O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14-22‰ compared with source water under our experimental conditions, and depleted by 4-7‰, compared with evaporative site water. This showed that it is the average lamina leaf water ¹⁸O / ¹⁶O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the ¹⁸O / ¹⁶O signatures of plant organic material and plant cellulose.